One known acceleration sensor formed by layering glass or silicon is described in JP-A-3-134570.
JP-B-4-55267 (equivalent to U.S. Pat. No. 4,679,434) discloses an acceleration sensor including a flat substrate having mounted thereon an acceleration detecting element and an electronic circuit for detecting a change in electrostatic capacitance of the acceleration detecting element. The substrate is mounted in a metal housing. The acceleration sensor is fixed in place by means of screws to be inserted through mounting holes formed through metal flanges in the housing.
The direction of acceleration to be detected by the acceleration sensor is perpendicular to the mounting surface of the acceleration sensor. Further, the use of the metal housing makes the acceleration sensor heavy. This structure results in various limitations on the mounting of the acceleration sensor in a system using the sensor.
In an air bag system, for example, a control unit of the air bag system is usually mounted in a horizontal direction of a vehicle, whereas the direction of acceleration to be detected by the acceleration sensor is the same as a longitudinal direction of the vehicle.
Accordingly, in such an application, because of the fact that the direction of acceleration to be detected by the acceleration sensor is perpendicular to the mounting surface of the acceleration sensor, it is necessary to mount the sensor perpendicular to the longitudinal direction of the vehicle. This necessitates complex mechanical arrangements for the mounting of the sensor.
Moreover, because of the relatively large weight of the acceleration sensor itself, the sensor is liable to mechanically resonate (which causes an error in detection of the acceleration).
JP-A-5-340963 discloses an acceleration sensor in which the acceleration detecting element is also mounted on a base plate such that the direction of acceleration to be detected is perpendicular to the base plate. The base plate is itself however mounted perpendicular to the metal base of the acceleration sensor housing, such that the direction of acceleration to be detected by the sensor is parallel to the mounting surface of the sensor. However, this construction still results in a complex and bulky device.
EP-B-0369352 discloses a capacitance-type accelerometer comprising three parallel silicon plates, the central one of which includes a movable electrode part. The plates are insulated from each other using thermal-oxide films in all areas between the plates except those adjacent the movable electrode. There is no discussion of a method of mounting the accelerometer.
It is an object of the present invention to provide an acceleration sensor which mitigates some of the above disadvantages.